Facebook users have been bombarded with explicit and violent images in the latest malware campaign aimed at the giant social networking site, a security researcher said Tuesday.

"For the last 24 hours, many people have reported seeing highly-offensive images on their Facebook news feeds," said Graham Cluley, a senior technology consultant at antivirus vendor Sophos, in an interview Tuesday. "But exactly how those images got there and what cause them to appear, is still somewhat of a mystery."

Users took to Twitter to express their outrage over the images, which Sophos said ranged from modified celebrity photos to pictures of extreme violence and animal abuse.

"Has anyone been on Facebook lately? My newsfeed looks like a porn site," said someone identified as Jay Ciroc on Twitter late Monday.

Cluley said that the evidence suggests the attack was not conducted by a rogue Facebook application -- one tactic criminals have used in the past -- but may have been a "clickjacking" campaign.

Clickjacking describes a type of attack where hackers plant invisible "buttons" on a website page; when a user clicks on the overlaying page component, they actually execute malicious code or script that can hijack their browser or personal computer.

The term was coined by researchers Robert Hanson and Jeremiah Grossman in 2008.

It's also possible, said Cluley, that malware already planted on PCs was responsible for the porn storm.

Facebook did not reply to a request for comment from Computerworld, but told other media outlets, including Reuters , that it was investigating the reports and would address the problem.

Other researchers have pointed to a specific piece of malware, however, that may be responsible.

According to Romanian security vendor BitDefender, the hacker collective known as "Anonymous" crafted a classic Facebook worm , codenamed "Fawkes Virus" last July, and had pledged to use it to celebrate Guy Fawkes Day, Nov. 5, a promise that was unfulfilled.

Guy Fawkes was arrested Nov. 5, 1605, for his part in the Gunpowder Plot to assassinate King James I of England. Anonymous has often used a mask of Fawkes as a logo for its disruptive hacking campaigns.

"The reaction has been very strong from Facebook users," said Cluley, who cited users who said it was the final straw, and that they would abandon Facebook until it got its security house in order.

That may be a while.

"Facebook has made improvements, but the scale of the problem they face is enormous, what with its 800 million members and the target that makes them," said Cluley. "I really, really hope Facebook can get a handle on spam and scams, but the spammers, the bad guys, are making just as much progress."